"good old Vancian".... I call that an oxymoron and the first subsystem anyone changed in AD&D.
WELL, remember, the ACTUAL magic system of
The Dying Earth is NOT the D&D magic system. Vance never talked about 'levels' or anything like that. Each wizard first of all knew certain spells, and they generally didn't share willingly. Secondly each one varied in his ability to memorize. Some could hold but one or two spells in memory, others a dozen. While there was a passage somewhere that hinted that some spells might be more difficult to memorize, there wasn't any statement that this resulted in a decrease in overall number of spells. It is equally likely it just meant some were out of the grasp of lesser casters, or they might miscast them (a distinct danger in his stories).
Casters also relied on IOUN stones to increase their casting capability and do other things. Presumably there might be other items with similar uses. Definitely there were many D&D-esque magic items that had single functions.
4e wizards are actually not a BAD model of Vancian wizards. I don't recall any at-will powers that Vance described, but many wizards did have captive magical beings that could perform certain magical tasks for them on command. Beyond that wizards tended to spend some time memorizing a selected list of spells, but its not clear that all of them required the same time, so you COULD consider encounter powers to be the 'easier' spells. I don't think it emulates Vance more accurately than the classic D&D magic system, but maybe not really WORSE either.
In any case, what I'm proposing wouldn't necessarily have level-based slots either, though that's also possible. Different rituals could also each require a different amount of time to cast, so some powers would be easier to refresh than others. A 30 second or 1 minute ritual would effectively represent an encounter power. A 1 or 2 hour one would be effectively daily.
Obviously this would diverge a good bit from the 4e power system too, in that there might not be a specific distribution of powers you'd have, 2 encounter, 2 utility, and 1 daily for instance. You might instead choose 2 dailies, 1 utility, and no encounter powers (or whatever). That would require some sort of 'slots' or at least careful DM time management. I'd go for slots personally. OTOH this is all obviated if you also use the idea of augments. In that case every power could be in all three flavors and you just spend 'vitality' to achieve the level of effect you desire.